Loudspeaker systems including those intended for residential two channel audio or multi-channel theater systems intend to embrace a substantial portion of the audio frequency range discernable by a listener. An important part of this range are low frequencies produced by relatively large loudspeaker transducers, generally known as woofers.
As with the mid and high-frequency parts of the audible range, it is known that the correct reproduction of musical pitch and timbre is strongly related to the attack part of the sound and less so to the decay part. The low frequencies are important in this regard because in all of occidental music the harmony is built upon the bass. If the reproduction of the bass frequencies has a slow attack, the overall sound is perceived as having an uncertain sense of pitch and a poor sense of rhythmic drive. It is thus of very great importance to design woofer systems which correctly render the attack part of the sound.
The correct rendering of the attack requires the ability for the motor of the loudspeaker to quickly accelerate the diaphragm. Since acceleration is proportional to force divided by mass it is necessary that the woofer transducer has a light moving system and a powerful motor. Conventionally designed woofer systems generally embody the opposite of these requirements. This is because there is a universal desire to make the woofer enclosure as small as possible. As will be discussed below, the stiffness of the air in the enclosure adversely modifies the characteristic of the woofer transducer, making optimization difficult at best and often impossible.
An excellent woofer system is shown schematically in FIG. 1. Woofer system 10 is comprised of cabinet 11 housing low frequency transducers 12 and 13. These low frequency transducers ideally operate in phase with each other whereby diaphragms 14 and 15 face each other being driven by motor assemblies 16 and 17. When low frequency transducers 12 and 13 are mounted opposite to one another as shown FIG. 1, large reaction forces associated with high power woofers located in cabinet structure 11 need not rely on mechanical grounding of the cabinet to the surrounding structures upon which the cabinet is placed.
In analyzing the low frequency transducer model of FIG. 1, one can create an electrical equivalent circuit (mobility analogy) of this assembly in free air. This is shown in FIG. 2A as a second-order resonant circuit with a natural frequency determined by the stiffness of the suspension and mass of the moving system. The amplitude (Q) of this resonance is determined by the damping due to mechanical loss. The resonance can be defined in terms of frequency and Q, and it constitutes a complex high-pass pole in the response of the loudspeaker.
Notwithstanding the above discussion, the electrical equivalent circuit shown in FIG. 2A does not tell the entire story. In this regard, reference is made to FIG. 2B. In this regard, when low frequency transducers 12 and 13 are placed within cabinet 11 which can be, for example, a sealed box, the stiffness of the air in the box is added to the stiffness of the suspension of the low frequency transducers and is shown as a parallel inductor. The consequence of this is that both the resonant frequency and Q are raised in value by approximately the square root of (1+(the stiffness of the speaker divided by the stiffness of the air in the box)). This can graphically be depicted by comparing FIGS. 2C and 2D.
A design goal of a woofer system is to maintain a low resonant frequency. Traditionally, this was done by increasing the moving mass (diaphragms 14 and 15), decreasing diaphragm stiffness or both. Stiffness has traditionally been decreased by making suspension components employed in such transducers more flexible or “limp” or by making enclosure 11 larger. Again, moving mass can only be increased by making diaphragms 14 and 15 heavier. However, adopting any of these traditional expedients represent a significant compromise as they tend to degrade performance of the woofer system. Softer suspension parts are not reliable, particularly if they are carrying a greater mass. Increased mass further requires a corresponding increase in motor strength if the ability to accelerate diaphragms 14 and 15 is to be maintained. A larger motor translates directly to higher production costs and a larger enclosure 11 may not be a suitable solution as cabinet size is generally considered to be a design constraint on any loudspeaker system. As a result, those engaged in loudspeaker design generally simply choose appropriately sized low frequency transducers, enclose them in an available volume and accept the resulting response.
It is thus an object of the present invention to provide a novel technique for dealing with the resonance of a low frequency transducer system.
It is yet a further object of the present invention to improve the operating range of a woofer system by providing an electrical circuit as an equal within the audio chain.
These and further objects will be more readily apparent when considering the following disclosure and appended claims.